1. FIELD OF THE INVENTION
The present invention relates to a hydraulic control device for an automatic transmission, in particular to an oil passage structure to a clutch hydraulic servo which engages at reverse running and at forward high speed running.
2 . DESCRIPTION OF THE PRIOR ART
Generally in a four speed automatic transmission constituted by a three speed automatic transmission mechanism and an under drive (or over drive) mechanism, a clutch which is engaged at reverse running (for example, a direct clutch) is also used in high speed running at D range (for example, a fourth speed at D range). A line pressure is applied to the clutch hydraulic servo by the same oil passage at both reverse running and forward high speed running.
However, the clutch hydraulic servo is under the high speed rotation when in forward high speed running, while the servo is under low speed rotation in reverse running, wherein the line pressure is invariably the same and is supplied through the same oil passage and the same orifice. Accordingly, if the orifice is designed for the forward high speed running in reverse running, hydraulic pressure is excessively applied to the hydraulic servo which is under low speed rotation, so that a shift shock may occur. On the other hand if the orifice is designed for the reverse running, in forward high speed running, hydraulic pressure is short due to leakage from a seal ring and a check ball for centrifugal hydraulic pressure release, so that the engaging time of the clutch becomes long, and further burning of the clutch plates may occur.